MGMAT's quant questions are not like the GMAT's quant; they're way too hard. GMATPrep is, of course, the one that most represents the real deal.
Princeton Review is pretty good. Kaplan's are too hard. Knewton also has a pretty good one.
Don't forget: GMATClub itself offers multiple tests as well (downside is Q too hard, V too easy)
Keep in mind that though tutoring centers, such as Princeton Review and Kaplan, will try to mimic the algorithm that the real GMAT uses. But there shouldn't be anyone out there who knows the exact algorithm. Having said that, most tests have been adjusted to how the test takers take them and then normalized to the curve of the real GMAT.
MGMAT's quant is way harder than those from GMAT, but the scores you get from
MGMAT are still a good indicator since most will miss those quant questions, thereby normalizing the scores using a huge sample size. My point is - the scores you get from any tutoring centers are good indicators, but also know to treat them with a grain of salt, since no one really knows the algorithm that's used to calculate the scores. Sorry...I think I'm on a huge tangent right now...lol